The present invention pertains to the art of metallurgy and materials science, and more particularly to a process for producing powder metallurgy or ceramic or glass objects of simple or complex geometries. The invention is particularly applicable to a process for joining powder metallurgy objects in the green or brown state to form larger or more complex parts. It will be appreciated that the invention may be advantageously employed in other environments and applications.
Although quite complex metallic shapes can be fabricated via a number of primary processes such as casting, forging, machining, and various powder-processing methods such as powder injection molding (PIM), all of these processes suffer some limitations in permissible part geometry. The limitations of the various forming methods, be they economic or technological, often dictate that a secondary joining operation is preferable or required.
A number of different techniques are currently known and used to join dense monolithic parts. These include: welding, brazing, soldering, reaction bonding, adhesive joining, and use of mechanical fasteners. However, such methods generally create non-uniform structures. For example, in the case of a weld, even though its composition may be nominally the same as the base metal, the joint material and/or "heat affected zone" of the base metal has a microstructure and concomitant properties that differ from the base metal, often substantially. In the case of soldering, brazing, and adhesive joining, a foreign material is left in the joint. Mechanical fasteners require holes that can serve as stress concentrators, and often the design must be constrained to allow access during assembly. These techniques, developed for dense monolithic materials, are also used currently for powder processed components after they are densified.
The three most prevalent methods for shaping parts are casting, deformation processing, and machining. In casting, the material is melted and poured into a mold. The liquid takes the shape of the mold cavity under some combination of gravity and pressure, and subsequent solidification results in the permanent storage of the shape information. In deformation processing, the material is typically heated to lower the effective yield stress and a shaped tool is brought to bear against the plastic mass under external pressure sufficient that permanent deformation occurs. The part typically retains its shape when the stress is removed. The familiar process of machining involves selective removal of material from the surface of a solid object by the action of a machine tool. In all of these processes, the metal is a dense solid monolith at the end of the shaping process. Powder processing, however, is different.
In powder processing, shaping is often mediated through the presence of a carrier fluid, which can be a water-based solution, mixture of organic liquids, or molten polymers. Metal, ceramic and glass powders can be processed with equal facility. The mixture can be made to emulate a liquid; a plastic, or a rigid solid by controlling the type and amount of carrier and the ambient conditions (e.g., temperature). The result of the shaping process is a "green" (i.e., unfired) powder compact that is a solid, but has an internal structure that consists of discrete powder particles held together by the action of a binder (usually a component of the carrier fluid). The powder compact is converted to a dense solid (and the microstructure is developed) through subsequent thermal processing to burnout, or pyrolize, the organic phase and densify, or sinter, the inorganic powder. An alternative method for densifying the part is to thermal process only to eliminate the binder and develop a modest amount of bisque strength followed by infiltration with a melt of a less refractory material. Both sintering and infiltration can be used with equal facility for powder metals, ceramics, and glasses.
The fact that powder processing involves two qualitatively different solid states offers the possibility of executing the joining process before processing has progressed to the point where the final microstructure of the material, and concomitant properties, are obtained. By working in the green state, the joining operation can form a bond through action on the organic binder, rather than directly on the metal or ceramic. A crucial constraint on any joining operation is that it be compatible with subsequent thermal processing and not interfere with densification. The state of a powder compact to be joined is dependent on the nature of its binder system which, in turn, is dictated by the need to be compatible with the primary processing operation. For example, many processes that employ a solvent-binder solution (e g, tape casting or gelcasting) produce compacts that are porous at the conclusion of a low-temperature drying step during or immediately after shaping. In cases where the binder is solidified (e.g., injection molding), the powder compact is usually nonporous. PIM feedstocks are generally composed of small inorganic particles dispersed in an organic medium. In the latter case, in particular, it is advantageous to employ binder systems that consist of a mixture of two or more materials. During binder removal, the process conditions are controlled such that one component of the binder is preferentially removed while the other remains in the compact. At this intermediate stage, the powder compact develops porosity and becomes functionally equivalent to a compact of the type produced with a solvent-based system. The presence of at least near-surface porosity is important for the joining process.
The process of the present invention is general and can be used with success to join two or more powder compacts regardless of the primary process used to define their shape. There are many situations in which there is a strong need to employ a joining operation, and representative solutions described herein are intended for illustration purposes only, and should not be viewed as limiting the scope of the invention.
The first representative situation concerns the production of large parts. For example, one technological limitation of the powder injection molding process has been the size limitation due to solidification shrinkage of the polymeric binder/carrier fluid. Typically, this limits the maximum thickness of molded objects to less than 1 inch. For small parts, PIM is a very attractive process, capable of great detail, geometric complexity, good material properties, good production rates, low generation of waste material and suitability to a wide variety of materials. But, for parts of large characteristic dimension or with highly variable cross section (e.g., a part with both thick- and thin-walled sections) it may be highly preferable to mold subcomponents and join them together after molding according to the process of the present invitation. Assembly of subcomponents also may allow design flexibility with a minimum investment in tooling costs. The application of the described process would allow joining small individually molded objects, with significantly simpler tooling, into one object, prior to sintering, in effect creating a larger object of uniform properties and microstructure.
A second representative situation involves green machining. Green machining, as implied by the term, is the process of machining powder compacts prior to binder burnout and sintering. Its advantages include low cost, high throughput, and material flexibility, because the material is softer in the green state and because machining behavior is determined by the nature of the binder rather than that of the powder particles. It is a widely used process. Green machining has geometric limitations that are analogous to those associated with conventional machining, i.e., complex concave surfaces can be very time intensive, require complex fixturing, and be costly. In addition, internal features may be completely inaccessible. Machining of subcomponents to be subsequently joined using the invention described herein will allow more complex parts to be machined, more economically.
A third representative situation is directed to Solid Freeform Fabrication (SFF). SFF is an emerging technology, often used for rapid prototyping, where solid objects are made without the use of traditional tools, such as molds and dies. In SFF, three dimensional computer models are stratified via computational software into separate layers which are used to direct layered-based manufacturing methods to form the three dimensional objects. One type of layered-based manufacturing uses thin sheets of material from which is cut the outline of each layer. Proper alignment and lamination of these layers produces a representation of the computer model. By utilizing a sheet formed of a carrier fluid and powder mixture, for example, powder injection molding feedstock, and using the joining method described herein, metallic or ceramic or glass objects of uniform structure may be fabricated. A second type of layered-based manufacturing uses thick layers of material and machine tools to form the geometry. By utilizing the process described herein, such layers, composed of a powder injection molding feedstock, may be joined creating a single object of uniform structure. Prior realization of the second method of rapid prototyping has been done by combining conventional brazing technology with machined dense metallic blocks.
The present invention contemplates a new and improved process which enables the fabrication of complex and non-complex objects, both metallurgical and non-metallurgical, using powder processing technology.